Krishna as Arjuna's Charioteer
Although Lord Krishna is the Supreme Personality of Godhead, out of His causeless mercy He was engaged in the service of His friend Arjuna. He never fails in His affection for His devotees, and thus He is addressed herein as infallible.
As charioteer, Krishna had to carry out the orders of Arjuna, and since He did not hesitate to do so, He is addressed as infallible.
Although Krishna had accepted the position of a charioteer for His devotee Arjuna, His supreme position was not challenged. In all circumstances, Krishna is the Supreme Personality of Godhead, Hrsikesa, the Lord of the total senses.
The relationship between the Krishna and His servitor is very sweet and transcendental. The servitor is always ready to render a service to the Lord, and, similarly, the Lord is always seeking an opportunity to render some service to the devotee.
Krishna takes greater pleasure in His pure devotee's assuming the advantageous position of ordering Him than He does in being the giver of orders.
As master, everyone is under Krishna's orders, and no one is above Him to order Him. But when Krishna finds that a pure devotee is ordering Him, He obtains transcendental pleasure, although He is the infallible master of all circumstances.